A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures.
In order to reduce the minimum printable size, imaging may be performed using radiation having a short wavelength. It has therefore been proposed to use an EUV radiation source providing EUV radiation within the range of 13-14 nm, for example. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.
Optical components for conditioning and focusing EUV radiation tend to be lossy, and much of the radiation is lost in the optical system. Further, available EUV radiation sources are limited in power. Consequently, production of high contrast images in resist material remains a challenge. A trade-off exists between the radiation dose required for good contrast, and the speed of exposure required for commercial productivity. Consequently, it is often more economic to produce device features by multiple exposures in a conventional (DUV) lithographic apparatus, rather than a single exposure in an EUV lithographic apparatus. As desired device features continue to shrink, however, the desire for the higher resolution of EUV lithography increases.